1. Field
The present invention relates to mitigating the effects of multipath error from indirect satellite signals in Global Navigation Satellite System (GNSS) devices.
2. Related Art
Navigation receivers that use global navigation satellite systems, such as GPS or GLONASS (hereinafter collectively referred to as “GNSS”), enable a highly accurate determination of the position of the receiver. The satellite signals may comprise carrier harmonic signals that are modulated by pseudo-random binary codes and which, on the receive side, may be used to measure the delay relative to a local reference clock. These delay measurements are used to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges are not true geometric ranges because the receiver's local clock is different from the satellite onboard clocks. If the number of satellites in sight is greater than or equal to four, then the measured pseudo-ranges can be processed to determine the user's single point location as represented by a vector X=(x, y, z)T, as well as to compensate for the receiver clock offset.
Positioning accuracy of GNSS technology is directly dependent on the accuracy of the delay measurements. A well known source of measurement error is due to multipath error. Multipath errors are caused by reflection of the radio signal from surfaces located near the receiving antenna. The antenna receives both the direct signal running the shortest path from the satellite to the receiver and the reflected signals following indirect paths. The combination of two (or more) signals at the antenna leads to the distortion of raw measurements. Multipath errors affect both pseudo-range and carrier phase measurements. The spatial correlation of multipath depends on the location of the reflection surface relative to the antenna. For surfaces generating reflections arriving at the upper side of the antenna, the carrier phase multipath may be substantially uncorrelated for antennas located only decimeters away.
A more detailed description of determining a location based on signals from GNSS satellites and potential errors associated with those signals is available in U.S. patent application Ser. No. 12/070,333, filed Feb. 15, 2008, and No. 12/360,808, filed Jan. 27, 2009, assigned to the assignee of the present invention, and each of which are incorporated herein by reference in their entirety for all purposes.
Therefore, a high precision, portable, complete handheld GNSS device that mitigates the effects of multipath errors is desired.